A quartz crystal microbalance is often used as part of a control system in order to optimize the production of specialty thin film coatings that may be used for a wide variety of functional and decorative uses. Exemplary uses of these film coatings cover a fairly wide spectrum that may include those aimed at increasing a machine component's wear resistance, narrow band optical filtering for increasing optical communication channels or spectral based chemical analysis, enhancement of light transmission through lenses and windows, and enhancing the reflection of mirrors and reducing the light reflected from a transparent surface or simply to apply a color; e.g. create a low cost substitute for gold color, among a myriad of other possible uses. The quartz crystal microbalance that is used for optimizing these processes is commonly known as a deposition process controller. A deposition process controller is used for real time sensing of the precise amount of material that is incident onto and subsequently adheres to a face of a resonantly vibrating piezoelectric crystal. The deposition parameters commonly sensed with this technology are the rate of deposition and total thickness. The crystal microbalance's sensor function relates to the frequency reduction of the composite resonator (monitor crystal plus deposited material) caused by the mass added to the vibrating piezoelectric plate that is partially exposed to the deposition. The foregoing application has a generally well-accepted formulation and is clearly described in chapter 2 of “Applications of Piezoelectric Quartz Crystal Microbalances” by Lu and Czanderna (Elsevier, first edition pp. 19-57) the entire contents of which are herein incorporated. In practice, the quartz crystal microbalance is located among or nearby the substrates that are being coated and is used as a surrogate substrate, allowing for precise inference of the amount of material added to the substrates by careful calculation based on the directional distribution of the material leaving a deposition source and use of the geometric relationships between the deposition source, substrates and the monitor crystal.
The precise and accurate measurement of the monitor crystal's frequency is an essential component of deposition control. The measurement of frequency is commonly accomplished according to two (2) basic techniques or methods. The first technique is referred to as the so called “active” method in which the monitor crystal and its adlayer becomes part of an oscillator circuit and the resulting frequency is measured by one of many possible ways of measuring frequency. One commonly employed method used to measure frequency employs an independent precision reference oscillator of known frequency to establish a precise and repetitive period of time by counting this oscillator's pulses to a predetermined fixed number. This process of repeatedly counting a fixed number of the precision oscillator's pulses establishes an exactly recurring period of time. A second counter is started and stopped by this recurring period that similarly measures the pulses from the resonating monitor crystal. This method of counting the number of measurement crystal pulses over a fixed period of time permits a very accurate measurement of the monitor crystal's frequency and is commonly known as the “period measurement technique”. The change of the monitor crystal's frequency is related to the mass added, so therefore knowing the density of the added material, the thickness may be inferred. Noting the change of thickness between successive recurring measurement periods, the rate of material deposition may then be calculated. This measured deposition rate is often used as the measured variable in a control system that manipulates the power to the deposition source such that the deposition rate can be stabilized or in some cases changed and manipulated in desired and advantageous ways.
The second common technique of determining the monitor crystal's frequency is considered a “passive” method that is described in detail, for example, in U.S. Pat. No. 5,117,192 to Hurd, the contents of which are herein incorporated. In the Hurd method, the monitor crystal is excited with a voltage of specific frequency and the related current response of the piezoelectric monitor crystal to this specific frequency is detected as being either capacitive, inductive or in-phase, the latter which is indicative of zero phase shift and infers the composite resonator (piezoelectric crystal plus coating(s) is operating at the desired series resonance point. Using the teachings of Hurd, the result of the phase measurement can be very rapidly converted to an intelligently calculated new applied interrogation frequency. Knowledge of the nature (i.e., sign and magnitude) of the monitor crystal's phase error response is used to calculate the next interrogation frequency, so that in only a few interrogation cycles the series resonance of the monitor crystal can be determined with very low error, even if material deposition is taking place and the frequency is simultaneously rapidly changing in response to the mass of material that is being added.
The measurement of frequency by each of the above methods are improved by minimizing sources of noise and insuring the circuits to and from the monitor crystal are low resistance, thereby further insuring the effective Q (i.e., the quality factor of the monitor crystal) can be maintained at a high level, during which as much deposition material may be added as is possible. When the effective crystal Q deteriorates, the measurement circuits are substantially less able to make a consistent frequency measurement and the control system is compromised by this source of noise. The adherence and growth of the deposition material on the monitor crystal is, by its nature, a dissipative process due to its lack of piezoelectric contribution, acoustic dissipation due to crystalline defects, and in many cases the introduction of tensile or compressive stresses to the composite resonator.
Essential requirements of a deposition monitor sensor constructed for commercial use include highly repeatable low noise and low resistance electrical connections, easy replacement of the monitor crystal, and product design of the sensor to insure that the deposition material being monitored and controlled is excluded from those areas that might compromise electrical isolation and integrity. The sharpness of the monitor crystal's resonance, which is related to the quality factor Q, is known to be reduced as the amount of deposition material is increased on the face of the monitor crystal and this loss of Q is known to increase the perceived noise of the measurement. When the noise reaches a level sufficient to make the measurement noise larger than that which can be tolerated by the process, replacement of the monitor crystal is necessitated. Any improvements taken to reduce the electrical resistance or shield the circuit elements from deposition or from other deterioration mechanisms, such as surface or interface corrosion, will have a positive effect on the measurement including lower noise and sometimes increased life of the monitor crystal.
To further clarify the needs outlined above, it must be understood that the electrical elements in the circuit used to apply the interrogation voltage waveform stimulating the monitor crystal and the subsequent sensing of the resulting current's phase relationship to that applied voltage waveform should have low resistance and the contacts and wires should be shielded from being coated by the material being applied. This is clearly required in the passive measurement technique and a necessary, but less obvious, requirement for any active measurement scheme. Loss of signal strength due to high resistance from loose or corroded connections or a parasitic electrical leak caused by conductive or capacitive leakage through deposited material from the applied radio frequency voltage and the return path allows a portion of the voltage to bypass the monitor crystal and is thereby detrimental to the optimal function of the measurement circuit.
In the prior art, the most common means of making electrical contact with a supported monitor crystal is to employ two separate spring contact assemblies or systems, often of the leaf type. Using two contact systems in series allows the user to have a convenient crystal holder package that can be simply and entirely removed from the deposition sensor for subsequent cleaning and monitor crystal replenishment without concern for the monitor crystal either falling out of a receiving cavity of the holder package, or otherwise tilting or hanging up and becoming broken during insertion into the receiving cavity.
In a known and typical dual contact scheme, a first leaf spring contact is used to make direct contact with one face (electrode) of a monitor crystal retained within a crystal holder and in which the first spring contact simultaneously pushes the monitor crystal onto an annular seat of the crystal holder. The first leaf spring contact is electrically connected to a conductive plate, allowing a second leaf spring contact, which is physically and electrically fixed to the deposition sensor body to be electrically connected to the first leaf spring contact when the holder assembly is physically inserted into the sensor body. Another intermediate conductive element then completes the electrical circuit to the detection/driving system of the deposition controller.
Clearly, a single leaf spring contact system having fewer pieces and contact junctions would be electrically superior, but without a retainer to hold the monitor crystal in proper relationship to its desired position in the crystal holder package all of the aforementioned practical problems associated with monitor crystal replacement are strongly manifested. When the positioning of the monitor crystal's face with the holder's crystal seat is not automatically aided by the local gravity field and instead the local gravity field tends to tilt or dislodge the monitor crystal, the installation or removal of the holder package becomes extremely difficult. One known early design that was successful in eliminating contacts, but was problematic regarding the replacement of the monitor crystal in hard to reach or gravity challenged positions, is typified in Lu's FIGS. 17a & b of the aforementioned book by Lu and Czanderna at page 53 thereof.
It is logical to assume that when any process is difficult and unpleasant, it is more likely to be performed improperly than when the process is easy and simple. For these reasons, it is common for most crystal sensor contact systems to include a retainer and accept the drawbacks of higher cost and slightly diminished electrical conduction properties in order to ease the above-referred to replacement task. The basic two contact system discussed herein has been employed successfully for more than 40 years.
The present invention minimizes the detrimental resistive effects of having numerous electrical contacts in series with the monitor crystal but without losing the convenient, orientation independent crystal holder package that is provided by using a retainer component. This desired electrical conduction benefit is manifested in a way that limits the potential for monitor crystal damage, while still providing secure placement of the monitor crystal within the holder package for insertion into the sensor that is orientation independent. The disclosed invention also provides a means of quickly and easily renewing the electrical contact system without hand tools or a need to solder in-situ. While it may be possible to add a retainer component to a design, such as that disclosed by Lu et al., it is found in practice that the spring contact used to make contact with the monitor crystal is susceptible to damage during routine cleaning; for example, a vacuum cleaning nozzle wiping across the sensor's holder cavity and reaching the contact spring causing distortion or breakage. The replacement of the contact spring requires tools, and or soldering and has to be performed in a position that is often difficult to reach or in an orientation that makes replacement and removal times long and frustrating. If soldering is required to repair the spring, the associated use of flux is a further complication because the applied flux must be thoroughly and meticulously removed after soldering and before processing can resume in order to avoid flux-caused contamination of the coating process and apparatus.
As a result, it can now be clearly seen that an invention that incorporates a retainer's function along with a means of reducing the number of electrical contacts, while fostering easy and quick in-situ replacement of any electrical contact system without tools or soldering is a very desirable improvement.
Therefore and according to one version, there is provided a combination retainer and electrical contact mechanism for a deposition monitor sensor, said mechanism comprising a sensor body and a monitor crystal retained within a crystal holder package. A removable flexible electrical contact spans between a fixed electrical contact element in the sensor body and a face of the retained monitor crystal. The mechanism further includes at least one insulating/isolating element in which the removable flexible electrical contact is associated with the at least one insulating/isolating element to provide a single mechanism.
In one version, the flexible electrical contact is defined by a coiled conductive spring having a first diameter section extending over a portion of its length and a second diameter section, which is larger than the first diameter section, defined over a separate portion. According to at least one version, the second diameter section is sized to engage a retention feature, such as an annular groove, formed within the insulating/isolating element.
The coiled conductive wire spring can be made from an electrically conductive wire. For example, the electrically conductive wire can be selected from the group consisting of stainless steel, piano wire, Inconel, beryllium copper, nickel copper and molybdenum, and in which each may be coated with gold or other contact enhancing material.
According to another embodiment, the removable flexible electrical contact can comprise a tubular body having leaf springs attached at respective ends thereof. The tubular body can include a first diameter section over a portion of its length and a second diameter section over another portion of its length, the second diameter section being sized for retention within an internal groove of the retainer. In one version, a split ring is disposed to engage the internal groove with the second diameter section.
In some versions, the retainer can be defined by a hollow cylindrical member having a split gap over its circumference, enabling the retainer to reduce its effective diameter when compressed. When the compressive force is removed, the retainer is configured to releasably engage an inner wall defining an axial bore of the crystal holder.
Alternatively, the retainer according to at least one embodiment can include a set of externally disposed ears that are configured to engage receiving slots defined in the crystal holder. The receiving slots include circumferential groove portions, enabling the retainer to be releasably secured to the crystal holder by rotating the retainer as engaged with the slot(s). As such, the retainer maintains a relatively light friction or interference fit within the machined bore of the crystal holder and provides a modest level of retention for the monitor crystal as the contained spanning electrical contact (e.g. spring) engages the crystal and creates friction.
The retainer can be made from an insulating material such as ceramic or a rigid plastic with acceptable process temperature and outgassing qualities, such as PEEK.
According to another version, there is provided a method for retaining a monitor crystal and providing electrical contact therewith for use in a deposition control monitor, said method comprising the step of providing a crystal holder having an axial bore and an annular seat sized for receiving a monitor crystal. According to this method, a retainer is disposed between the crystal holder and a sensor body, the retainer being at least partially disposed in the axial bore of the crystal bore and the sensor body including a fixed electrical contact engaged with an electrical source of the deposition control monitor. A releasable flexible spanning electrical contact is engaged between a fixed electrical contact of the sensor body and one face of a retained monitor crystal, wherein at least one of the crystal holder and the retainer includes at least one feature for maintaining the spanning electrical contact in a fixed orientation.
In one embodiment, the removable flexible spanning electrical contact includes a first diameter section over a portion of its length and a second diameter portion over another portion of its length. The second diameter section is configured to engage an internal groove provided in the retainer.
In some embodiments, the flexible electrical contact is a coiled conductive spring having respective ends configured to engage a fixed electrical contact of the sensor body and the face of the retained monitor crystal, respectively. In another version, the flexible electrical contact is defined by a tubular conductive body having leaf springs attached at opposing ends of the tubular body.
In some versions, the retainer can be defined by a hollow cylindrical member having a split gap over its circumference, enabling the retainer to reduce its effective diameter when compressed. When the compressive force is removed, the retainer is configured to releasably engage an inner wall defining an axial bore of the crystal holder.
Alternatively, the retainer according to at least one embodiment can include a set of externally disposed ears that are configured to engage receiving slots defined in the crystal holder. The receiving slots include arcuate or circumferential grooved portions, enabling the retainer to be releasably secured to the crystal holder by means of a twisting action once engaged. As such, the retainer maintains a relatively light friction or interference fit within the machined bore of the crystal holder and provides a modest level of retention for the monitor crystal.
One advantage provided by the herein described combination contact/retainer system is that fewer components are required, reducing the overall number of components as well as related costs in manufacture and replacement. The reduction in the total number of parts and at least some machined features are eliminated, thereby creating a much simpler and more reliable apparatus.
Another related advantage is an overall reduction in the number of electrical interfaces in the monitor crystal sensor, which reduces the voltage drop due to contact resistance.
In addition, maintenance of sensors is simplified herein by eliminating the need to solder at least some replacement items, eliminate removal and replacement of screws, and avoiding the necessity to remove the entire sensor assembly from the vacuum coating tool to do this.
Still further, convenience of positive retention of the monitor crystal within the crystal holder assembly is provided, thereby easing the process of removal and replacement of monitor crystals by eliminating concern for having to make contact with the monitor crystal's surface and minimizing the possibility of breakage of the monitor crystal while performing these routine operations.
In addition, a crystal retainer is introduced that can provide the function of restraining the monitor crystal in the crystal holder, while allowing a contact spring device to pass through the holder unimpeded.
These and other features and advantages will be readily apparent from the following Detailed Description, which should be read in conjunction with the accompanying drawings.